This invention relates to electrical feedthrough bushings of the type used for providing electrical connection to within the interior of power transformer assemblies. The bushings generally comprise an electrical conductor within a porcelain enclosure containing a quantity of paper insulating material and mineral oil for cooling and insulation purposes. The outer surfaces of the porcelain housing are provided in a radially extending plurality of skirts for generating the longest possible electrical path length for a reasonably short overall linear extent. When porcelain is used as the outer jacket material a quantity of dirt accumulates on the porcelain surface over extended periods of use. During conditions of high humidity and during periods of gentle rain the portions of the porcelain surface directly subjacent the extending skirts remain reasonably dry whereas the outer exposed surfaces of the skirt contain small accumulations of water. When the bushings are used in high voltage applications it is sometimes possible for arcing to occur between the wet portions of the porcelain assembly such that an arc discharge can eventually occur between the top and bottom terminals of the bushing causing the bushing to become inoperative.
The current state of the bushing art utilizes an electrically conductive glaze coating on the outer surface of the porcelain outer jacket to provide a high resistance current path between the bushing terminals. The purpose of the conductive coating is to prevent an excess build-up of charge on the porcelain surface and to generate sufficient heat to deter the accumulation of moisture. Upon continued exposure to the elements the chemical and electrical properties of the conductive glaze may become altered to such an extent that the conductive glaze is no longer capable of providing the outer surface high resistance current path. At this point the bushing behaves similar to a plain porcelain insulated bushing and summarily becomes subjected to the moisture conditions described earlier.
The purpose of this invention is to provide a means for electrically monitoring the condition of the conductive glaze coating in order to determine whether the bushing is operating in the conductive glaze mode or as a conventional bushing.